Reptile featured in Aesop regularly (3)
I believe the answer is:
asp
'reptile' is the definition.
(asp is a kind of reptile)
'aesop regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'aesop' are 'ASP'.
'featured in' is the link.
(Other definitions for asp that I've seen before include "A glider's what's called for" , "Joe Blake is Australian species" , "Snake of the Nile" , "snake appears" , "the snake in Cleopatra''s bosom" .)